An absorption cooling system uses an absorbent solution and an absorption refrigerant combination to produce the cooling effect of the system. A generator heats the absorbent solution to drive the refrigerant from the solution, producing a refrigerant vapor. The refrigerant vapor is then cooled in a condenser to condense the refrigerant. The liquid refrigerant then flows to an evaporator. In the evaporator, the pressure on the liquid refrigerant is relieved, allowing it to evaporate at a lower temperature. The refrigerant draws heat into the system as it evaporates to produce the cooling effect of the absorption cooling system.
The refrigerant vapor then flows to an absorber. In the absorber, the refrigerant vapor is absorbed into the absorbent solution from the generator. After the refrigerant is absorbed into the solution, the absorbent solution returns to the generator and the cycle repeats.
An absorption cooling system generator is typically powered by natural gas, oil, or steam. The system typically operates at subatmospheric pressure. At subatmospheric pressure, the generator requires less heat to drive the refrigerant from solution, the refrigerant will evaporate in the evaporator at a lower temperature, and the system requires less energy input to produce its cooling effect.
However, an absorption cooling system faces a formidable problem when operated at subatmospheric pressure--air may leak into the system. The main component of air, nitrogen, is not condensible. Another source of noncondensible gas is chemical reactions within the system. The components of the system react with the absorbent solution to produce hydrogen, a noncondensible gas.
Noncondensible gas, from any source, hinders the performance of the system. Noncondensible gas migrates to the lowest pressure point in the system in the absorber. There, the gas may blanket off a portion of the absorber heat transfer surface and prevent reabsorption of the refrigerant into solution. Also, noncondensible gas increases the pressure of the system. Typically, the evaporator and the absorber share a common housing and pressure. If noncondensible gas is allowed to accumulate, the evaporator pressure may increase to the point that the refrigerant will not evaporate at the desired temperature. Accordingly, noncondensible gas must be removed or "purged" from the system.
In the prior art, noncondensible gas has been removed from the system in a variety of ways. Noncondensible gas has been removed from the absorber by pumps, siphons, aspirators, and other devices. These methods often require a complex system of fall tubes or aspirators.
In many prior art systems, the system cannot purge noncondensible gas unless the system is running. Many prior art systems utilize a diverted flow of absorbent solution or cooling water to operate the purge system. In these systems, the purge system is inoperable without a flow of these fluids.
However, noncondensible gas may collect in the system when it is shut down. Absorption cooling systems are typically maintained at subatmospheric pressure when the system is shut down and noncondensible gas may leak into the system. The noncondensible gas impedes start up of the system and adversely affects performance of the system until the noncondensible gas is collected and purged.
In many prior art systems, the noncondensible gas is collected in a purge tank. Many tanks must be purged manually. In other systems, the purge system interrupts the normal operation of the absorption cooling system.
Some prior art absorption cooling systems remove noncondensible gas from the absorber and collect the gas at absorber pressure. The gas is often collected in a storage tank before purging. These systems require either a large storage tank or frequent purging because of the relatively low absorber pressure.
The systems that collect noncondensible gas from the absorber must also transfer noncondensible gas in the condenser to the absorber. Often, these systems simply employ a large orifice between the condenser and the evaporator so that both condensed refrigerant liquid and noncondensible gas flow to the evaporator and eventually the absorber. These systems, however, have difficulty maintaining the necessary pressure differential between the condenser and the evaporator. In other cases, the system must employ a complex system of siphons or fall tubes to transfer the noncondensible gas from the condenser to the evaporator.
In an absorption cooling system, noncondensible gas is typically intermixed with refrigerant vapor. Therefore, to prevent loss of the refrigerant from the system, the noncondensible gas must be separated from the refrigerant. In the prior art, the noncondensible gas and intermixed refrigerant vapor have been bubbled through the absorbent solution to absorb the refrigerant vapor. Alternatively, the refrigerant vapor has been condensed on a condensation surface and separated from the noncondensible gas, which does not condense. In some systems, the system coolant fluid is used as a heat sink to condense the refrigerant vapor and separate it from the noncondensible gas. In other systems, the atmosphere is used as a heat sink. For optimum performance, however, a purge refrigeration system may be provided to condense and separate the refrigerant vapor.
U.S. Pat. No. 5,031,410, issued to Plzak, discloses a purge refrigeration system for a centrifugal chiller. In Plzak, the purge refrigeration system is a discrete refrigeration system that employs a refrigerant different from the main cooling system refrigerant. The purge refrigeration system evaporator is located in a purge tank. The purge tank is connected to the condenser of the main cooling system so that the refrigerant vapor and the noncondensible gas in the main cooling system may freely flow into the purge tank. In the purge tank, the refrigerant vapor is condensed on the evaporator of the purge refrigeration system and separated from the noncondensible gas. As the refrigerant vapor condenses, the purge refrigerant is warmed. The condensed refrigerant is returned to the main cooling system and the noncondensible gas is collected in the purge tank.
As the noncondensible gas collects in the purge tank, the noncondensible gas displaces the refrigerant vapor and blankets the purge refrigeration system evaporator. Accordingly, the purge refrigerant is no longer warmed by the refrigerant vapor. The purge refrigerant temperature is monitored by a temperature sensor. At a specified temperature detected by the temperature sensor, the noncondensible gas is pumped from the purge tank. The purge tank again fills with refrigerant vapor and the cycle repeats.
However, Plzak discloses a purge apparatus for use on a centrifugal chiller. A centrifugal chiller differs greatly from an absorption cooling system. For example, a centrifugal chiller uses a single refrigerant, such as R11, rather than a refrigerant and solution combination. A centrifugal chiller uses a compressor rather than a generator and an absorber. Also, a centrifugal chiller is typically powered by electricity rather than gas, oil, or steam. In addition, a centrifugal chiller operates at significantly higher pressures than an absorption cooling system. The condenser of an absorption cooling system operates at only approximately 1 p.s.i.a. (0.69 N/cm.sup.2). In contrast, the condenser in a centrifugal cooling system operates at approximately 20 p.s.i.a. (14 N/cm.sup.2). Accordingly, the challenges of purging a centrifugal chiller differ greatly from the challenges of purging an absorption cooling system.
First, as previously described, the noncondensible gas in an absorption cooling system must be transferred to a collection point. Also, the absorption cooling system operates at a significantly lower pressure. The lower pressure of the absorption cooling system makes it more difficult to draw noncondensible gas and absorption refrigerant vapor from the condenser to the purge tank. Also, when the absorption refrigerant is water, the temperature of the purge refrigerant may not be too low or the absorption refrigerant will freeze in the purge tank. To prevent freezing but still maintain the necessary temperature and pressure differential, the purge refrigerant must be maintained in a narrow temperature range.
Accordingly, it is an object of the present invention to provide a purge apparatus and method for removing noncondensible gas from an absorption cooling system that operates at subatmospheric pressure.
A further object of the present invention is to provide a purge apparatus and method that draws noncondensible gas from an absorber, directs the gas to the condenser, and purges the gas from the higher pressure condenser so that the volume of noncondensible gas to be purged is reduced.
Yet another object of the present invention is to provide a purge apparatus and method that separates noncondensible gas from intermixed refrigerant vapor by condensing the refrigerant vapor on the evaporator of a discrete purge refrigeration system.
An additional object of the present invention is to provide a purge apparatus and method that automatically senses when noncondensible gas should be purged from the system and automatically purges the gas.
A still further object of the present invention is to provide a purge apparatus and method that can track the purge rate so that air leaks in the system can be detected and corrected.
Another object of the present invention is to provide a purge apparatus and method that operates when the system is either operating or shut down.
Finally, an object of the present invention is to provide a purge apparatus and method that may be easily retrofitted to existing absorption cooling systems.